Method for coating a metal strip and equipment for implementing said method

ABSTRACT

The invention relates to a process for coating a metal strip, in which a layer of an oxidizable metal or an oxidizable metal alloy or a metal oxide is vacuum-deposited on a metal strip precoated with zinc or with a zinc alloy, the coated metal strip is then coiled, and the wound coil undergoes a static diffusion treatment so as to obtain a strip having a coating that comprises, in the upper portion, a layer of an alloy formed by diffusion of the oxidizable metal or the oxidizable metal alloy in all or part of the zinc or zinc alloy layer, and also to equipment for implementing the process.

The present invention relates to a process for coating a metal strip,more particularly intended for coating steel strip with layers based onzinc and on oxidizable metal elements, without in any way being limitedthereto.

Various methods for depositing metal coatings composed of a metal layer,several successive layers of different metals, or metal alloys, on ametal surface such as a steel strip are known. Among these, mention maybe made of hot-dip galvanizing, electroplating and various vacuumdeposition processes (evaporation, magnetron sputtering, etc.)

Certain products have to be deposited as several layers, fortechnological or even economic reasons, and have to undergo a diffusionheat treatment for obtaining the alloy with the desired properties. Thismay for example be the case for zinc-magnesium coatings that mayadvantageously replace pure zinc coatings or coatings of other zincalloys.

The diffusion heat treatment may prove to be complicated and expensive.It may involve the use of large quantities of inerting gas in order toprevent oxidation reactions promoted by the high temperatures during theheat treatment. Furthermore, to avoid any risk of oxidation between thedeposition of the oxidizable element and its diffusion treatment, it isnecessary to carry out the two operations one immediately after theother, without exposing the strip to the open air.

Furthermore, a continuous heat treatment line runs at speedsincompatible with the time required for the diffusion.

A first solution would consist in producing continuous treatmentequipment operating at moderate temperature, the length of the equipmentallowing the time necessary for the diffusion to take place, but thisequipment is then bulky and expensive and there may not always be enoughroom to install it on existing production lines. Thus, trials have shownthat, for a zinc coating covered with a layer of magnesium with athickness of 1.5 μm, 50 seconds at 300° C. are required foraccomplishing the diffusion, which represents a length of 150 m to bemaintained at temperature for a strip running at 180 m/min.

Equipment of such size is not easily acceptable, and, in industrialpractice, it is therefore necessary to envisage higher temperaturesusing shorter continuous treatment equipment. Thus, for a zinc coatingcovered with a layer of magnesium with a thickness of 1.5 μm, it ispossible to limit the diffusion time to around 10 seconds, whichrepresents a length of 30 m to be maintained at temperature for a striprunning at 180 m/min. However, the working window for this type ofdynamic diffusion treatment is very narrow since, as soon as 350 to 360°C. is reached, the coating melts, passing through a eutectic phase,resulting in the properties of the coating being impaired. Operating theprocess on high-capacity lines in which the strip runs at 160 to 180m/min is therefore very tricky. Furthermore, the energies involved arehigher and the treatment of certain grades of steel, such asbake-hardening grades, widely used in the automobile industry, isprecluded, since their properties would be impaired by such a heattreatment.

Moreover, even by reducing the size of the equipment, the lengths oftreatment to be carried out still remain such that the continuoustreatment equipment must provide strip support rollers or stripdeflectors, which are complicated and expensive, since they have to becooled and designed so as not to degrade the layer formed, somethingwhich may in particular happen when a strip sticks on the rollers.

The object of the present invention is therefore to remedy the drawbacksof the processes of the prior art by providing a process formanufacturing a metal strip covered with a coating based on zinc or on azinc alloy and an oxidizable metal or an oxidizable metal alloy, whichconsumes little energy and little or no inerting gas, which is easy toimplement, which is compact and which allows metal substrates of varioustype to be treated.

For this purpose, a first subject of the present invention is formed bya process for coating a metal strip, in which a layer of an oxidizablemetal or an oxidizable metal alloy is vacuum-deposited on a metal stripprecoated with zinc or with a zinc alloy, the coated metal strip is thencoiled, and the wound coil undergoes a static diffusion treatment so asto obtain a strip having a coating that comprises, in the upper portion,a layer of an alloy formed by diffusion of the oxidizable metal or theoxidizable metal alloy in all or part of the zinc or zinc alloy layer.

The process according to the invention may also comprise variousoptional features, taken individually or in combination:

the coating comprises only a layer of an alloy formed by diffusion ofthe oxidizable metal or the oxidizable metal alloy throughout the zincor zinc alloy layer;

the coating comprises a lower portion, consisting of zinc or a zincalloy, and an upper portion consisting of a layer of an alloy formed bydiffusion of the oxidizable metal or the oxidizable metal alloy into aportion of the zinc or zinc alloy layer;

the metal strip has been precoated with zinc or a zinc alloy by ahot-dip galvanizing process;

the metal strip has been precoated with zinc or a zinc alloy by anelectroplating process;

the metal strip has been precoated with zinc or a zinc alloy by a vacuumdeposition process;

the metal strip is precoated with a zinc or zinc alloy layer having athickness of between 0.5 and 15 μm, preferably between 0.5 and 7.5 μmand more particularly preferably between 0.5 and 5 μm;

the zinc or zinc alloy coated metal strip is coated with magnesium or amagnesium alloy by vacuum deposition;

a magnesium layer is deposited by vacuum evaporation with a thickness ofbetween 0.2 and 5 μm, preferably between 0.2 and 2 μm;

a layer of an alloy with the composition Zn₂Mg, possibly comprisingZn₁₁Mg₂ compounds, is formed during the static diffusion annealing;

the coating on the metal strip coated with the oxidizable metal or metalalloy is oxidized on the surface before undergoing said static diffusiontreatment;

the coil of metal strip is heated for a time of between 4 and 40 hoursat a temperature below 200° C.;

the metal strip is a steel strip and may be made of a bake-hardeningsteel.

A second subject of the invention is formed by equipment forimplementing the process according to the invention, comprising:

a device for galvanizing said metal strip; followed by

a vacuum deposition coating device; and

a static heat treatment device operating in a controlled atmosphere.

The equipment according to the invention may also comprise the followingvariants, taken individually or in combination:

the galvanizing device is a hot-dip galvanizing device;

the galvanizing device is an electrogalvanizing device;

the galvanizing device is a vacuum deposition galvanizing device.

Other features and advantages of the invention will become apparent onreading the following description, given solely by way of example.

The process according to the invention applies more particularly, butnot solely, to the treatment of steel strip coated with zinc or with azinc alloy. The term “zinc alloy” denotes any compound comprising atleast 50% zinc and possibly containing, for example, aluminum, iron,silicon, etc.

The coated strip may be obtained by any galvanizing process, whetherhot-dip galvanizing, electroplating or vacuum evaporation deposition,for example. However, strip coated by electroplating or by vacuumevaporation deposition is preferred in which the coating thickness isconstant over the entire surface of the steel coil.

The thickness of the coating will preferably be between 0.5 and 15 μm.This is because below 0.5 μm, there is a risk that the corrosionprotection of the strip is insufficient. The thickness of the coatingmay be up to 15 μm, depending on the end applications of the strip, butin general it is less than 7.5 μm as it is unnecessary to go beyond thisto have the level of corrosion resistance required in particular in theautomobile industry.

Of course, the process according to the invention may be used with anycoated metal substrate not liable to have its properties irreversiblyimpaired during the subsequent heat treatment. Thus, the processaccording to the invention may especially be applied to bake-hardeningsteel strip containing large amounts of carbon in solid solution, whichmust not be completely precipitated before the strip undergoes a formingoperation, by stamping or any other suitable process. The heat treatmentaccording to the invention, despite the low temperature levels thereof,will precipitate a small portion of the carbon in solid solution presentin these grades, but a surface work hardening (skin-pass) treatmentafter diffusion will restore the properties of these grades, somethingwhich would not be possible with the processes of the prior art. Bycarrying out the static annealing cycle at low temperature it is thuspossible to make the heat treatment compatible with most metallurgies.

The zinc or zinc alloy coated metal strip is firstly coated with a layerof an oxidizable metal or an oxidizable alloy by a vacuum depositionprocess. Mention may especially be made of magnetron sputtering, coldplasma deposition and vacuum evaporation processes, without theinvention being in any way limited thereto.

The use of such a process makes it possible in particular to deposit avery thin layer of oxidizable metal or metal alloy, preferably with athickness of between 0.2 and 5 μm. Furthermore, such a coating processallows this additional layer to be deposited without heating the stripand therefore without subjecting it to any inopportune diffusion betweenthe substrate and the zinc layer.

The deposition of the oxidizable metal or metal alloy is conventionallycarried out starting with a metal coil, which is unwound before runningit through the deposition chamber. The strip runs through this chamber,in which the coating is deposited, then exits the chamber and is coiled,again conventionally.

The oxidizable metal may in particular consist of magnesium, which hasthe advantage of greatly enhancing the corrosion resistance of a metalstrip when it is added to the zinc in the surface coating of this metalstrip. In most applications, the magnesium thickness may be limited to 2μm owing to this considerable improvement in corrosion resistance.

After this deposition step, the metal strip is therefore covered with azinc or zinc alloy layer above which is a layer of oxidizable metal oralloy. Since the strip is coiled and then stored without it beinginerted, the outermost surface of said strip rapidly oxidizes on contactwith oxygen in the air, thus forming an oxidation layer.

The present inventors then attempted to carry out a static annealingoperation on the non-unwound metal coil, which made it possible for theoxidizable element to diffuse completely correctly into the upperportion of the zinc or zinc alloy layer. Quite surprisingly, theoxidation layer in no way impeded this diffusion, contrary to what aperson skilled in the art might have expected.

Furthermore, this very oxidation layer proved to be propitious inpreventing the turns of the coil from sticking together during thediffusion heat treatment.

Of course, it is still possible to protect the metal strip fromoxidation during transportation and storage between the coatingoperation and the heat treatment by depositing a final protection layer.However, the laboratory trials carried out have shown that thisprotection is unnecessary.

The static annealing is carried out in conventional box annealingequipment in an atmosphere which may be an oxidizing or a nonoxidizingatmosphere.

In particular, the present inventors have shown that the annealingtreatment in an oxidizing atmosphere, such as air, prevents certainheterogeneities in surface color of the strip from appearing.

As in the case of the metallurgical annealing of an uncoated metal coil,the rates of temperature rise and fall must be adapted according to thetemperature heterogeneities accepted within the metal coil. The othercharacteristics of the heat cycles to be carried out, such as the timeduring which the temperature rises, the soak time and the duration ofcooling, are also determined according to the desired maximumtemperature level. Thus, FIG. 1 shows an example of an actual thermalcycle for the treatment of a steel coil weighing 2 tons. This figureindicates the temperature settings and the control of the burners for astatic heat treatment device. It may be seen that the temperature riseis accomplished over 14 hours, until 170° C. is reached, after which theheating is stopped and the coil gradually cooled down to 55° C. after atotal treatment time of 30 hours.

Since the duration of this type of annealing ranges generally from 4 to40 hours, the maximum temperatures reached will generally be below 200°C. This makes it possible to treat a large number of steel or metalgrades that are sensitive to an excessively high temperature rise andthat could not undergo continuous annealing. This is because, owing tothe high run speed during a continuous annealing operation, the holdtemperature would be much higher.

Static diffusion cycles were carried out at various temperatures andwith various levels of tightening between turns. These various levels oftightening have shown that, well beyond the normal range of windingstresses and well beyond the normal range of pressures undergone duringheat treatments on coils, there was no sticking between turns.

The sheets resulting from these static diffusion heat treatments revealcorrectly diffused products and the formation of the intended alloy atthe surface of the coating, with either complete or partial diffusiondepending on the case.

EXEMPLARY EMBODIMENTS Example 1

A 15-tonne coil of bake-hardening steel strip coated with a 2.5 μm layerof zinc by electroplating was then coated with a 1 μm layer of magnesiumby vacuum evaporation. The strip then spent several days in the open airwith no particular protection, resulting in the formation of a magnesiumoxide layer on the outermost surface.

The metal strip was then subjected to a static annealing treatment at160° C. to make the magnesium diffuse into the zinc.

The inerting gas used during the static annealing was anitrogen/hydrogen mixture, identical to that used conventionally forannealing low carbon steel. No oxidation of the metal coil was observedduring the heat treatment because a nonoxidizing inerting gas was used.

FIG. 2 shows the thermal cycle undergone by the coil. This figure alsoshows the variation in the temperature of the gas inside the furnace,the variations in temperature of the various points in the steel coil,including in particular the hottest point and the coolest point.

It may be seen that the temperature rise is carried out over a period ofapproximately 14 hours. The hold at the 160° C. soak temperature lastedabout 2 hours, during which the magnesium diffusion took place. Inpractice, such a soak hold is obtained by simply turning the furnace offand leaving the coil therein. The cooling down to a temperature of 70°C. lasted 8 hours. Thus, the total cycle time was about 24 hours. Tostart the cooling, the coil was removed from the furnace and placedunder a cooling bell, enabling these cooling conditions to becontrolled.

Thus, all of the magnesium was alloyed with a portion of the zincforming the first layer, and a coating having a zinc sublayer and aZn—Mg alloy upper layer was obtained. The differences in temperaturebetween the hottest point of the coil and the coolest point of the coilrepresent a difference in diffusion rate limited to a few percent,resulting in no significant modifications to the properties of thecoating. It is also possible to adapt the temperature soak time beforecooling so as to allow this diffusion rate to be made completelyuniform, should this prove necessary.

Example 2

In the same way as in example 1, two 15-tonne coils A and B ofbake-hardening steel strip identical to that used above, these beingcoated with a 2.5 μm layer of zinc by electroplating, were then coatedwith a 1 μm layer of magnesium by vacuum evaporation. Each strip wasthen left in the open air without any particular protection, causing theformation of a magnesium oxide layer on the outermost surface.

Each metal strip was then subjected one after another to a staticannealing treatment at 160° C. in the same static annealing equipment,to make the magnesium diffuse into the zinc. The thermal cycle undergoneby the coils was the same as in example 1, the only treatment differencebeing the type of atmosphere chosen for annealing coil B.

Coil A:

The gas used during the static annealing of coil A was an inertnitrogen/hydrogen mixture identical to that mentioned in example 1. Thesame effects as regards oxidation of the metal coil during the heattreatment could be observed.

In addition, whereas in example 1 the color of the metal strip after thestatic annealing treatment was a uniform light gray, in this case darkerhalos appeared on the edges of the strip. The color along the axis ofthe strip remained unchanged relative to that observed in example 1.Electron microscope observations showed that, in the light zone, themagnesium crystallites present on the surface of the strip had verysharp and well-defined hexagonal geometric shapes. In contrast, in thedark zones, the magnesium crystallites present on the surface of thestrip were deformed and had irregular edges.

Complementary investigations did not reveal any appreciable differencefrom a chemical standpoint between the light zone and the dark zones.

This slight color heterogeneity is therefore very probably due to thedeformation of the magnesium crystallites on the surface of the strip inthe dark zones: these crystallites scatter light differently and producethe observed visual effect.

Since all the parameters in the study (steel strip, zinc and magnesiumcoatings, thermal characteristics and atmosphere of the annealing) hadbeen left unchanged in relation to those of example 1, the cause of thecrystallite deformation proves to be due to the annealing equipmentitself. Specifically, depending on the production campaigns in progress,the equipment may contain variable amounts of polluting species, such asfor example carbon residues coming from rolling oil combustion. Inaddition, the shape of the coloration defect, in the form of anoscillation starting from the edges, leaves one to suppose that there isan effect caused by the inter-turn diffusion of gaseous speciescontained in the annealing atmosphere. Thus, the presence of pollutingagents in the annealing atmosphere, combined with a thermal gradienteffect, transverse to the strip, could explain the observed phenomenon.

Coil B:

The gas used during the static annealing of coil B was air. The observedeffects as regards oxidation of the metal coil during the heat treatmentwere the same as those observed in the case of coil A and in example 1.

However, in this case, the coloration defect did not appear. The striphad a uniform light gray color, identical to that observed in example 1.Moreover, electron microscope observations showed that the magnesiumcrystallites present on the surface of the strip had very sharp andwell-defined hexagonal geometric shapes, as in the light zone of coil A.

Since coils A and B were treated one after the other in the sameannealing equipment, the sole unique element of the latter experimentwas the annealing atmosphere, all the other parameters remainingunchanged (steel strip, zinc and magnesium coatings, thermalcharacteristics of the annealing). The fact that the heat treatment wascarried out in air therefore had the effect of neutralizing thethermochemical effects responsible for the appearance of the colorationdefect observed on coil A (elimination of the pollutants, etc.).

It has thus been shown that, in the case of a diffusion heat treatmenton industrial equipment, the fact of using an oxidizing gas such as airinstead of an inert gas as annealing atmosphere makes it possible toneutralize the thermochemical effects due to the presence of pollutants(arising for example from the previous uses of the equipment) andpossibly leading to the appearance of a coloration defect on the finalproduct. It is therefore possible in particular to carry out the processaccording to the invention without inerting.

The invention makes it possible to produce coatings comprising alloys bydepositing multilayers, without having to invest in a complicated andexpensive on-the-run diffusion device. The space required in the coatingline is therefore around 50% less than for equipment for carrying outthe annealing in line. The invention is therefore particularly suitablefor new products to be applied on an existing production line when theproduction volume is small or when the start-up curve is long and slow.

The invention uses a static heat treatment of longer time and at lowertemperature than has to be applied on a continuous line. The inventionallows compact tools to be used to carry out the diffusion. Itminimizes, or even eliminates, the consumption of inerting gas andminimizes the energy consumption per ton (and the power to be installed)by lowering the temperature of the diffusion cycle, thereby making thetreatment compatible with a wide range of steel products and steelgrades.

The invention allows the use of box annealing equipment or similarproduction tools to carry out the alloying necessary for producing theend product. By using existing box annealing tools it is possible toreduce the investment cost by around 30% (taken over an investmentincluding the deposition and the heat treatment) and thus to decide toinvest in, and launch on the market, new products of shorter life timeor lower cumulative volume.

By combining processes, it is possible to produce innovative multilayercoatings. Combined with a diffusion heat treatment, these may give riseto an alloy giving the coated product advantageous surfacecharacteristics.

It is then necessary either to construct a new production plant or tosupplement an existing production plant. The second case is moreopportune provided that it combines, in the alloy, a metal alreadydeposited using the existing equipment and provided that the equipmentis available, in terms of space and capacity, for providing the newproduction.

The present invention is aimed in particular at obtaining zinc-magnesiumcoatings, but it is not limited to these coatings—it encompasses anycoating based on an oxidizable metal or oxidizable alloy.

1. A process for coating a metal strip, comprising vacuum-depositing a layer of an oxidizable metal or an oxidizable metal alloy on a metal strip precoated with zinc or with a zinc alloy, coiling the coated metal strip, and treating the wound coil with a static diffusion treatment so as to obtain a strip having a coating that comprises, in the upper portion, a layer of an alloy formed by diffusion of the oxidizable metal or the oxidizable metal alloy in all or part of the zinc or zinc alloy layer.
 2. The process as claimed in claim 1, in which the static diffusion treatment is carried out on the wound coil in an oxidizing atmosphere.
 3. The process as claimed in claim 1, in which the static diffusion treatment is carried out on the wound coil in a nonoxidizing atmosphere.
 4. The process as claimed in claim 1, in which the metal strip has been precoated with zinc or a zinc alloy by a hot-dip galvanizing process.
 5. The process as claimed in claim 1, in which the metal strip has been precoated with zinc or a zinc alloy by an electroplating process.
 6. The process as claimed in claim 1, in which the metal strip has been precoated with zinc or a zinc alloy by a vacuum deposition process.
 7. The process as claimed in claim 1, in which the metal strip is precoated with a zinc or zinc alloy layer having a thickness of between 0.5 and 15 μm.
 8. The process as claimed in claim 1, in which the zinc or zinc alloy coated metal strip is coated with magnesium or a magnesium alloy by vacuum deposition.
 9. The process as claimed in claim 8, in which a magnesium layer is deposited by vacuum deposition with a thickness of between 0.2 and 5 μm.
 10. The process as claimed in claim 8, in which a layer of an alloy with the composition Zn₂Mg, possibly comprising Zn₁₁Mg₂ compounds, is formed during the static diffusion treatment.
 11. The process as claimed in claim 1, in which the coaling on said metal strip coated with the oxidizable metal or metal alloy is oxidized on the surface before undergoing said static diffusion treatment.
 12. The process as claimed in claim 1, in which said coil of metal strip is subjected to a diffusion heat treatment for a time of between 4 and 40 hours by heating it at a temperature below 200° C.
 13. The process as claimed in claim 1, in which the metal strip is a steel strip.
 14. The process as claimed in claim 13, in which the metal strip is made of a bake-hardening steel.
 15. Equipment for manufacturing a metal strip coated by the process as claimed in claim 1, comprising: a device for galvanizing said bare metal strip; a vacuum deposition coating device; and a static heat treatment device operating in a controlled atmosphere.
 16. The equipment as claimed in claim 15, in which said galvanizing device is a hot-dip galvanizing device.
 17. The equipment as claimed in claim 15, in which said galvanizing device is an electrogalvanizing device.
 18. The equipment as claimed in claim 15, in which said galvanizing device is a vacuum deposition galvanizing device. 